The present invention relates to a synthetic fused silica glass and a method for the preparation thereof. More particularly, the invention relates to synthetic fused silica glass containing hydroxyl groups only in an extremely low content of 1 ppm or below and having a remarkably high viscosity at high temperatures so as to be useful as a crucible for the crystal growing of semiconductor silicon single crystals by the Czochralski method and other tools employed in the processing of semiconductors as well as to a so-called sol-gel method for the preparation of such high-grade fused silica glass.
As is well known, various kinds of semiconductors or, in particular, high-purity silicon in the form of a single crystal are obtained by growing the single crystal in the so-called Czochralski method from a melt of the semiconductor substance contained in a crucible. The material of the crucibles is mostly fused quartz glass which is conventionally prepared by melting natural quartz crystals. One of the problems in natural quartz glass, when an article, e.g., crucible, made from such glass is used in semiconductor processing, is that the glass unavoidably contains various kinds of metallic impurities which badly affect the quality of the semiconductor products, e.g., silicon single crystals, grown by using such a crucible resulting in a decreased yield of acceptable semiconductor products. Accordingly, it is a key requirement that the material of the articles such as crucibles used in the semiconductor processing has a purity as high as possible.
In view of the above mentioned problem relative to the purity of fused quartz glass, it is a trend in recent years that crucibles for single crystal growing of silicon made from natural quartz are increasingly under replacement with those made from synthetic fused silica glass. Several different methods are known in the prior art for the preparation of such high-purity synthetic fused silica glass including the so-called flame hydrolysis method in which a silicon-containing volatile compound such as silicon tetrachloride is pyrolytically hydrolyzed in an oxyhydrogen flame to give silica particles which are then fused and vitrified into fused silica glass. One of the most serious disadvantages in this flame hydrolysis method is that the thus produced fused silica glass usually contains a large amount of hydroxyl groups in an amount sometimes exceeding 1000 ppm with a consequently low viscosity of the glass at high temperatures and eventual foaming of the glass when it is melted in vacuum.
As a modification of the above described pyrolysis of a volatile silicon compound, a method of using a plasma flame in place of the oxyhydrogen flame has been proposed. This method of plasma pyrolysis, however, is not suitable as an industrial process due to the high cost for the plasma flame generation and the difficulties sometimes encountered when the method is applied to mass production of fused silica glass.
Apart from the above described pyrolytic methods, a method recently highlighted is the so-called sol-gel method in which a hydrolyzable silicon compound such as ethyl orthosilicate is hydrolyzed in a hydrolysis medium to prepare a silica sol which is subsequently converted into a gel. Fused silica glass is obtained by calcination and fusion of the silica gel after drying. This sol-gel method, however, does not provide a full improvement in respect of the content of hydroxyl groups in the fused silica glass if not to mention the disadvantage due to the relatively long time taken for the method. Despite the above mentioned problems, the sol-gel method is preferred as a method for the preparation of fused silica glass for use as a material of optical fibers, photomask plates for lithographic processing of ICs, optical lenses and the like owing to the nearly perfect amorphousness of the glass obtained by the method.
Several attempts and proposals have been made in the prior art for the improvement of the properties of the fused silica glass to be used in the above mentioned optical applications as prepared by the sol-gel method including a method taught in Japanese Patent Kokai 62-241837 in which fine silica particles obtained by the hydrolysis of an alkyl silicate in the presence of a basic catalyst in the hydrolysis medium are blended with a silica sol obtained by the hydrolysis of an alkyl silicate in the presence of an acidic catalyst, a method taught by D. W. Scheere et al. in Journal of Non-crystalline Solids, volume 63, pages 163-172 (1984) in which silica particles obtained by the oxidation of silicon tetrachloride are dispersed in chloroform containing n-propyl alcohol as a dispersing aid and the dispersion is then gelled by using ammonia vapor, and so on.
Application of fused silica glass obtained by the sol-gel method to non-optical refractory silica glass articles, such as crucibles, is also under investigation. For example, Japanese Patent Kokai No. 63-166730 teaches a method in which silica particles obtained by the hydrolysis of a hydrolyzable silicon compound by using an acidic catalyst in the presence of an alkaline ingredient such as sodium are first transformed into a crystalline form of .alpha.-cristobalite before the silica is melted to give fused silica glass.
The sol-gel method in the prior art in general has disadvantages in respect of the relatively low productivity as an industrial process and large energy consumption required for removing a large amount of the solvent used as the hydrolysis medium. The above mentioned method taught in Japanese Patent Kokai No. 62-241837 is not suitable as a method for the preparation of fused silica glass having a high viscosity at high temperatures comparable to that of natural quartz glass so that the glass prepared by the method cannot be used as a material of articles used in high-temperature processing of semiconductors. The method described in the above mentioned Japanese Patent Kokai No. 63-166730 has a disadvantage in respect of the difficulty in removing the sodium impurity so that the glass obtained by the method cannot be used as a material for the heat treatment of semiconductors although the glass may have a sufficiently high viscosity at high temperatures